In the assembly of semiconductor packages, greases, fillers and adhesives are used to attach covers and heat sinks to completed dies and to attach different parts of a package to each other. Because a semiconductor die heats with use, the different parts of a package, including the die will expand and contract. Any adhesive must allow for this expansion and contraction. As a result, greases and polymers are often used between parts. On the other hand, heat must be conducted away from the die so that it does not overheat during use. Greases and polymers are very poor heat conductors, but most heat conductors do not accommodate expansion and contraction between parts of the package.
Thermal interface materials (TIMs) are used to attach heat spreaders to a die and to attach heat sinks to a package. TIMs are designed to balance adhesion, flexibility, heat conductance, thermal stability, ease of use, and cost, among other factors. A variety of different formulations have been developed for different applications that feature different characteristics.
In order to make electronic devices smaller and faster, dies of different types and sizes are installed into the same package. These types of packages may be referred to as a system in a package (SiP), a multiple die module (MDM), or by other names, depending on the particular features and configuration of the die combination. A whole multiple die system may be integrated into a single die or only related components may be combined, such as a processor and local memory or an image processor and a camera controller.